1. Field of the Invention
The present invention relates generally to pallet assemblies of sandwich-type composite structure having a cellular core and, in particular, to such pallet assemblies whose structure is reinforced locally.
2. Background Art
Sandwich-type materials having cellular cores have very important characteristics resulting from their being light in weight yet very rigid.
Conventionally, such a panel is constructed by sandwiching a cellular core having low strength characteristics by gluing it or bonding it between two skins, each of which is much thinner than the cellular core but has excellent mechanical characteristics.
The patent document FR 2 711 573 discloses a method of making a panel of sandwich-type composite structure having a cellular core. In that method, said panel is made in a single step by subjecting a stack to cold-pressing in a mold, which stack is made up of at least a first skin made of a stampable reinforced thermoplastics material, of a cellular core made of a thermoplastics material, of a second skin made of a stampable reinforced thermoplastics material, and of a first external covering layer made of a woven or non-woven material, the skins being preheated outside the mold to a softening temperature.
Such a method is particularly advantageous because of the fact that it makes it possible, in a single operation, both to generate cohesion between the various layers of the composite structure, and to shape the panel.
The resulting panel conserves all of the mechanical properties imparted by the cellular core sandwich structure.
European patent EP 0 649 736 B1 explains the principle of molding substantially flat parts out of thermoplastic sandwich material (TSM). The part is made in a single stage by pressing in a cold mold, at a pressure in the range of 10 bars to 30 bars, a stack consisting of at least a first top skin layer of stampable reinforced thermoplastics material, a cellular or honeycomb core of thermoplastics material and a second bottom skin layer of stampable reinforced thermoplastics material. The axes of the cells of the cellular core are generally oriented perpendicular to the skin layers. The skin layers and core are previously heated outside the mold to a softening temperature. Such sandwich material is also described in U.S. Pat. No. 5,683,782. The cellular core of such material enables the part to be very rigid while being light in weight.
U.S. Pat. No. 6,050,630 discloses a molded composite stack including a cellular core for a vehicle and a mold for forming the stack into a vehicular part, such as a floor panel.
Panels of sandwich-type composite structures having a cellular core have strength characteristics sufficient to enable mechanical structures subjected to large stresses to be reinforced structurally without making them too heavy. Such panels are in common use in shipbuilding, aircraft construction, and rail vehicle construction.
However, the non-uniformness of the mechanical stresses to which they are subjected sometimes makes it necessary to form local reinforcing plies at those places in said panels where the mechanical stresses are greatest.
In the field of aircraft construction, sandwich structure composite panels are made that are based on thermosettable resins reinforced with glass fibers.
In order to impart the desired shapes to the panels, and to maintain the shapes, the glass fibers and the thermosettable resin (in the form of pre-pregnates) are deposited layer-by-layer in a mold, and are then heated to high temperatures so as to cure (i.e. polymerize) the resin permanently.
The molds used may have a punch or a die, or else both a punch and a die.
Making such locally-reinforced panels consists firstly in defining zones where stresses are concentrated in the resulting panels, such zones being defined either by real testing or by computer simulation, and then in adding reinforcing plies at those places so as to make it possible to withstand such stresses.
The reinforcing plies are one-directional mats or woven fabrics of glass fibers, of carbon fibers, or of natural fibers embedded in a thermosettable resin, with an orientation that is determined by the orientation of the stresses. They are cut out to a pattern using special machines, e.g. water-jet cutting machines.
The reinforcing plies are disposed layer-by-layer in a mold, either manually or by means of a robot, with each ply having its own orientation.
That operation may be referred to as the xe2x80x9claying upxe2x80x9d operation.
Then comes the baking step which is the longest step of the method of making such pieces because the stack of layers must be heated sufficiently to cure the thermosettable resin.
The various layers disposed in the mold are pressed in the mold by evacuating the mold. Such evacuation serves to press the materials against the die or the punch, and to remove surplus resin.
The desired shape is thus obtained with the fibers being impregnated with the resin as well as possible.
That xe2x80x9claminationxe2x80x9d technique, and in particular the xe2x80x9claying upxe2x80x9d operation, is characterized by a very low level of automation, and a large labor input.
Although, by means of the concept of localizing the strength, that technique makes it possible to achieve performance levels that are high for the pieces that are made in that way, it requires rigorous monitoring of quality.
As a result, that technique is very costly and cannot be used at the high production throughputs implemented in many fields such as the automobile industry.
Generally, plastic pallets can be easily molded and are lighter in weight than wooden pallets. Furthermore, in general, plastic pallets are more durable than wooden pallets as shown in U.S. Pat. No. 5,497,709.
Plastics processing technology has enjoyed significant recent advances, such that traditional high-strength materials such as metals are being replaced with fiber composite materials. These materials are not only light, but also are flexible and durable.
U.S. Pat. Nos. 5,891,560 and 6,165,604 disclose fiber-reinforced composites prepared from a depolymerizable and repolymerizable polymer having the processing advantages of a thermoset without being brittle. Impregnation of polymer into the fiber bundle is achieved, while still producing a composite with desirable physical properties and high damage tolerance.
One factor that has limited the number of plastic pallets is that plastic pallets require a given amount of relatively expensive plastic material for a desired measure of pallet strength. U.S. Pat. Nos. 5,868,080 and 6,199,488 disclose reinforced plastic pallet constructions and assembly methods wherein multiple reinforcing bars are employed. The reinforcing bars preferably comprise composite structural members of fiberglass reinforced thermosetting plastic fabricated from a pultrusion process.
As noted in the above-mentioned ""560 and ""604 patents, although thermoset composites have excellent mechanical properties, they suffer from several disadvantages: thermoset matrices have relatively limited elongation, the thermoset precursors are a source of undesirable volatile organic compounds (VOCs), the composites cannot be reshaped or recycled, and their production rates are limited.
Consequently, in principle at least, thermoplastic composites would solve many of the problems associated with thermosets. For example, unlike thermosets, thermoplastics can be reshaped, welded, staked, or thermoformed. Furthermore, thermoplastics are generally tougher, more ductile, and have greater elongation than thermosets.
An object of the present invention is to provide a reinforced composite pallet assembly of the cellular core sandwich-type which can be made simply and cheaply, and can be implemented at high throughputs.
In carrying out the above object and other objects of the present invention, a reinforced composite pallet assembly of the sandwich type having a cellular core is disclosed. The assembly includes a substantially flat deck having front, back and side edges and includes: a) a load-bearing skin made of a reinforced thermoplastics material; b) an upper grid of reinforcing slats; c) a cellular core made of a thermoplastics material; d) a lower grid of reinforcing slats; and e) a tine-engaging skin made of a reinforced thermoplastics material. The upper and lower grids of reinforcing slats are positioned symmetrically with respect to a plane formed by the cellular core at predetermined places against the skins and the cellular core and the shape of the deck being obtained from a single pressing stage. At least one support supports the deck so that tines can lift and support the pallet at the tine-engaging skin.
Slats of each of the grids may be positioned adjacent to the front, back and side edges of the deck and may extend from positions adjacent the front, back and side edges of the deck to a center of the deck.
The deck may include at least one outer covering layer made of a woven or non-woven fabric disposed on the load-bearing skin and covering the front, back and side edges of the deck.
The assembly may further include another deck wherein the at least one support interconnects the decks at their tine-engaging skins and wherein the assembly is at least stackable or rackable.
The other deck may include an outer covering layer made of a woven or a non-woven fabric disposed on its load-bearing skin and covering its front, back and side edges.
The single pressing stage may have a forming pressure for forming the deck which lies in the range 106 Pa to 3xc3x97106 Pa.
While the deck is being formed, the skins may have a forming temperature lying in the range approximately 160xc2x0 C. to 200xc2x0 C.
The skins may be made of a woven fabric or mat of glass fibers and of a thermoplastics material.
The reinforcing slats of the grids may be made of reinforced thermoplastic composite.
The composite may be fiber-reinforced and may include a depolymerizable and repolymerizable thermoplastic polymer resin.
The resin may be a thermoplastic polyurethane.
The thermoplastics material of the skins may be a polyolefin and is preferably polypropylene.
The cellular core of the deck may have an open-celled structure of the tubular or honeycomb cell type, constituted mainly of polyolefin and preferably polypropylene.
A plurality of spaced supports may support the deck adjacent corners of the deck.
The at least one support may be made of a thermoplastics material and wherein substantially the entire pallet assembly is recyclable.
The assembly may further include an electronic identification device to allow the assembly and its corresponding load to be identified.
The above object and other objects, features, and advantages of the present invention are readily apparent from the following detailed description of the best mode for carrying out the invention when taken in connection with the accompanying drawings.